We are studying the genetic control of developmental processes by focusing on a cluster of genes in Drosophila which controls the state of differentiation achieved by certain body segments of the fly. We are attempting to determine not only how each gene of the cluster evokes specific developmental changes in the fly but also how the genes of the cluster are themselves regulated and coordinated in such a way that development proceeds in an orderly manner. We are proposing, in collaboration with E. Davidson, to isolate and characterize the DNA from the bithorax gene cluster. In addition we are planning to synthesize special duplication-bearing strains to study the effect of artificially increasing the amount of genetic redundancy for the bithorax region and to use in generating somatic mosaicism for certain mutant genes, especially those which result in lethal effects when not rescued by wild-type tissue. The overall objective is to use the bithorax system to understand how higher organisms, including human beings, regulate their growth and development at the genetic level. BIBLIOGRAPHIC REFERENCES: Garcia-Bellido, A. and Lewis, E.B. (1975) Autonomous cellular differentiation of homoeotic bithorax mutants of Drosophila melanogster. Develop. Biol. In press. Lewis, E.B. (1975) Possible genetic consequences of successful treatment of childhood tumors with ionizing radiation. Radiology 114:147.